Olefin polymerization catalysts are of great use in industry and polyolefins are widely used commercially because of their robust physical properties. Hence, there is interest in finding new catalyst systems that increase the marketing value of the catalyst and allow the production of polymers having improved properties.
For example, various types of polyethylenes, including high density, low density, and linear low density polyethylenes, are some of the most commercially useful. Suitable polyolefins can be prepared with a catalyst that polymerizes olefin monomers. The ability to precisely modulate polymer architecture and composition is a long standing goal within the field of polymer synthesis which makes catalyst engineering a crucial point. Therefore, there is a need for catalysts having high activity and capable of forming polyolefins, for example, with high molecular weight and high comonomer content. The faculty to precisely control at the atomic levels the structure of a catalyst through manipulation of both the metal and, perhaps more importantly, the supporting ligand(s) is central to the development and utilization of such catalysts in a myriad of applications.
Over the past few years, there has been continuing interest in using bridging ligands in the synthesis of polynuclear complexes of paramagnetic transition metal ions, especially ligands which contain potentially bridging nitrogen donor atoms, called amine bisphenolate ligands. Notwithstanding the apparent simplicity of the ligand framework, amine bisphenolate ligands are valuable tools for generating metal complexes with an adequate balance of thermal stability and reactivity. Although a considerable number of ligands bearing C, N, P, or S donor-based functionalities have been synthesized, Metal IV catalyst complexes containing amine bisphenolate ligands having O-donor fragments have been much less studied. In this context, the ability of amine bisphenolate trianionic ligands for supporting high-oxidation-state metal complexes holding vacant coordination sites, which have been recently exploited for a range of catalytic transformations, is exceptional. There is no literature precedent on group IV catalyst complexes containing a tridentate ethylene bridged amine bis(phenolate) ligand framework for olefin polymerization and the complexes described in the present disclosure are the first examples of this type of catalyst structure.
Low density polyethylene is generally prepared at high pressure using free radical initiators, or in gas phase processes using Ziegler-Natta or vanadium catalysts. Low density polyethylene typically has a density in the range of 0.916 g/cm3 to 0.940 g/cm3. Low density polyethylene produced using free radical initiators is known in the industry as “LDPE”. LDPE is also known as “branched” or “heterogeneously branched” polyethylene because of the relatively large number of long chain branches extending from the main polymer backbone. Polyethylene in the same density range, e.g., 0.916 g/cm3 to 0.940 g/cm3, which is linear and does not contain long chain branching, is known as “linear low density polyethylene” (“LLDPE”) and can be produced by conventional Ziegler-Natta catalysts or with metallocene catalysts. “Linear” means that the polyethylene has few, if any, long chain branches, referred to as a g′vis value of 0.97 or above, such as 0.98 or above. Polyethylenes having still greater density are the high density polyethylenes (“HDPEs”), e.g., polyethylenes having densities greater than 0.940 g/cm3, and are generally prepared with Ziegler-Natta catalysts or chrome catalysts. Very low density polyethylenes (“VLDPEs”) can be produced by a number of different processes yielding polyethylenes having a density less than 0.916 g/cm3, such as 0.890 g/cm3 to 0.915 g/cm3, or 0.900 g/cm3 to 0.915 g/cm3.
Polyolefins, such as polyethylene, which have high molecular weight, generally have desirable mechanical properties over their lower molecular weight counterparts. However, high molecular weight polyolefins can be difficult to process and can be costly to produce. Polyolefin compositions having a bimodal molecular weight distribution are desirable because they can combine the advantageous mechanical properties of a high molecular weight fraction of the composition with the improved processing properties of a low molecular weight fraction of the composition.
Useful polyolefins, such as polyethylene, may have a comonomer, such as hexene, incorporated into the polyethylene backbone. These copolymers provide varying physical properties compared to polyethylene alone and can be produced in a low pressure reactor, utilizing, for example, solution, slurry, or gas phase polymerization processes. Polymerization may take place in the presence of catalyst systems such as those employing a Ziegler-Natta catalyst, a chromium based catalyst, or a metallocene catalyst. The comonomer content of a polyolefin (e.g., wt % of comonomer incorporated into a polyolefin backbone) influences the properties of the polyolefin (and composition of the copolymers) and is influenced by the polymerization catalyst.
A copolymer composition, such as a resin, has a composition distribution, which refers to the distribution of comonomer that forms short chain branches along the copolymer backbone. When the amount of short chain branches varies among the copolymer molecules, the composition is said to have a “broad” composition distribution. When the amount of comonomer per 1,000 carbons is similar among the copolymer molecules of different chain lengths, the composition distribution is said to be “narrow”.
Like comonomer content, the composition distribution influences the properties of a copolymer composition, for example, stiffness, toughness, environmental stress crack resistance, and heat sealing, among other properties. The composition distribution of a polyolefin composition may be readily measured by, for example, Temperature Rising Elution Fractionation (TREF) or Crystallization Analysis Fractionation (CRYSTAF).
Polyolefin compositions may have broad composition distributions that include a first polyolefin component having low molecular weight and low comonomer content while a second polyolefin component has a high molecular weight and high comonomer content. Compositions having this broad orthogonal composition distribution (BOCD) in which the comonomer is incorporated predominantly in the high molecular weight chains can provide improved physical properties, for example toughness properties and environmental stress crack resistance (ESCR).
Also, like comonomer content, a composition distribution of a copolymer composition is influenced by the identity of the catalyst used to form the polyolefins of the composition. Ziegler-Natta catalysts and chromium based catalysts generally produce compositions with broad composition distributions, whereas metallocene catalysts typically produce compositions with narrow composition distributions.
Nonetheless, polyolefin compositions formed by catalysts capable of forming high molecular weight polyolefins typically also have a broad molecular weight distribution (MWD), as indicated by high polydispersity indices, and/or the polyolefins are of such high molecular weight (e.g., Mw of 1,500,000 or more) as to have processing difficulties due to hardness. Furthermore, catalysts capable of forming high molecular weight polyolefins typically have low activity (e.g., amount of desirable polymer produced per a period of time).
References of interest include: Dornow, A., Petsch, G., Archiv der Pharmazie und Berichte der Deutschen Pharmazeutischen Gesellschaft (1951), 284, 153; Amdts, D., Loesel, W., Roos, O., Ger. Offen. (1993), DE 4220353 A1 19931223; and Majima, K., Tosaki, S., Ohshima, T., Shibasaki, M., Tetrahedron Lett., 2005, 46(32), pp. 5377-5381; WO 2013/101476; WO 2013/101478; CN 105418672.